Don't worry about your son entering high school in 10th grade. Some high schools in the US only run from 10th to 12th grades anyhow - and they'll do their best to transfer his school records over with some sort of equivalence.

The main advantage of taking AP courses is if he is planning on attending university in the US. Doing well in the AP exams at the end of the program can result in him receiving credit for first year courses at the university level in many US universities.

The International Bac is recognized by most US universities, too, but depending on exactly where you'll be located in the US, it may be necessary to find a private school that offers the IB. I've heard of a few public schools with an IB program, but they aren't the norm by any means. Obviously, the IB would be a big advantage if he wants to attend university back in Belgium or anywhere in Europe.
Cheers,
Bev